Dwarf lions of different species can sometimes coexist, as long as they are not too similar. But you'll probably want at least one active swimmer. Aim for something larger than the lion. So, for a Dwarf Lion, an exquisite wrasse (my personal centerpiece fish), rabbit fish (also venomous, can have a theme!), dwarf angels (make sure to get a mature one! Also not reef safe), etc. are all good options. If you don't have fish that can be bullied, a school of damsels would do well with a dwarf lion, though some might end up food for a volitans lion.
Tangs do well with Lions, even full sized lions, but not in a 55. If you want a really wicked cool fish that is super ugly (beauty an the beast, anyone?) how about an angler? Saw one at the LFS
, and the darn thing had ELBOWS!